Direct Monitoring of Microgel Formation during Precipitation Polymerization ofN-Isopropylacrylamide Using in Situ SANS

Abstract

Poly( N -isopropylacrylamide) microgels have found various uses in fundamental polymer and colloid science as well as in different applications. They are conveniently prepared by precipitation polymerization. In this reaction, radical polymerization and colloidal stabilization interact with each other to produce well-defined thermosensitive particles of narrow size distribution. However, the underlying mechanism of precipitation polymerization has not been fully understood. In particular, the crucial early stages of microgel formation have been poorly investigated so far. In this contribution, we have used small-angle neutron scattering in conjunction with a stopped-flow device to monitor the particle growth during precipitation polymerization in situ. The average particle volume growth is found to follow pseudo-first order kinetics, indicating that the polymerization rate is determined by the availability of the unreacted monomer, as the initiator concentration does not change considerably during the reaction. This is confirmed by calorimetric investigation of the polymerization process. Peroxide initiator-induced self-crosslinking of N -isopropylacrylamide and the use of the bifunctional crosslinker N , N ′-methylenebisacrylamide are shown to decrease the particle number density in the batch. The results of the in situ small-angle neutron scattering measurements indicate that the particles form at an early stage in the reaction and their number density remains approximately the same thereafter. The overall reaction rate is found to be sensitive to monomer and initiator concentration in accordance with a radical solution polymerization mechanism, supporting the results from our earlier studies.

Synthesis of Poly(N-vinylcaprolactam)-Based Microgels by Precipitation Polymerization: Pseudo-Bulk Model for Particle Growth and Size Distribution

“…Virtanen et al (2019) recently concluded a comparable formation mechanism based on in situ small-angle neutron scattering measurements of the synthesis of PNIPAM-based microgels. 46…”

Acrylamide precipitation polymerization in a continuous flow reactor: an in situ FTIR study reveals kinetics

“…The deviation between the conversion and the particle growth kinetics might occur due to reacted monomer not being built into microgel particles, but as a part of oligomers. The absolute values of the rate constants and the total conversion strongly depend on the respective reaction conditions and can be influenced by the monomer properties [66], the addition of accelerators [50], or by surfactants [40]. Furthermore, the progress implies an offset from the origin, indicating the existence of an induction period which could not be accessed here.…”

“…In the context of inline monitoring, prominent vibrations in acrylamides have been used as probes for monomer conversion in batch syntheses, using Raman spectroscopy [49]. Moreover, Virtanen et al [50] studied the microgel formation by in situ SANS in a stopped-flow setup.…”

The Role of the Initiator System in the Synthesis of Acidic Multifunctional Nanoparticles Designed for Molecular Imprinting of Proteins

“…It can be noted at first glance that the polymerization rate using the redox initiator systems (NP3 and NP5) is much higher at 40 °C than with merely APS at 60 °C (NP1 and NP2). This is attributable to the much faster initiator decomposition rate in the redox system compared to its thermal homolysis [38,39]. With SBS and TEMED maximum conversion is achieved already after about 40 minutes, while using only APS with and without SDS approximately 120 minutes is needed.…”